The thermal emission of gases in a plume can be measured by a Fourier-transform spectrometer that is located some distance from the plume. In order to measure quantitatively the amount of a particular gas of interest, in general a large spectrally structured background must be removed. Differencing techniques, in which a measured background spectrum is subtracted from a measured spectrum believed to contain a target, often do not remove background spectral features adequately. The inadequacy of two-spectrum differencing techniques is due to the spatial and the temporal variations in a scene. We present a method by which to reduce spatial and temporal spectral clutter to instrument random noise, allowing the measurement of gas amounts in an effluent plume. The method is applied to simulated data and field data to show its effectiveness.

Nonlinear least squares spectral curve fitting has been used to derive vertical mixing ratio profiles for NO2 and H2O above 16 km from high resolution (∼.02 cm−1) solar spectra collected during sunset with a balloon borne interferometer. The NO2 profile shows a sharp peak of ˜8 ppbv at 32 km falling rapidly to <0.5 ppbv at 17 km. The H2O profile shows a broad peak of ˜6.5 ppmv at 30 km falling to <4 ppmv at 17 km.

Abstract. This paper presents the three-waveband spectrally agile technique (TWST) for measuring cloud optical depth (COD). TWST is a portable field-proven sensor and retrieval method offering a unique combination of fast (1 Hz) cloudresolving (0.5 • field of view) real-time-reported COD measurements. It entails ground-based measurement of visible and near-infrared (VNIR) zenith spectral radiances much like the Aerosol Robotic Network (AERONET) cloud-mode sensors. What is novel in our approach is that we employ absorption in the oxygen A-band as a means of resolving the COD ambiguity inherent in using up-looking spectral radiances. We describe the TWST sensor and algorithm, and assess their merits by comparison to AERONET cloud-mode measurements collected during the US Department of Energy's Atmospheric Radiation Measurements (ARM) TwoColumn Aerosol Project (TCAP). Spectral radiance agreement was better than 1 %, while a linear fit of COD yielded a slope of 0.905 (TWST reporting higher COD) and offset of −2.1.

The essential elements of practical nonlinear least squares analysis of atmospheric absorption spectra are discussed. These include speed of analysis, flexibility of spectrum type, and stability in the presence of noise. A computer program which embodies these elements is described, and two types of results are presented. The first is a series of least squares analyses of synthetic spectra, both noise free and noisy. The second is a summary of results from analyses of high resolution solar spectra. Among the parameters which have been fit are trace gas mixing ratios, temperatures, spectral background, instrument resolution, phase error and channel spectrum parameters, line shifts, intensities, and broadening coefficients.

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